1. Field of the Invention
The invention relates to a transparent, non-heat-sealable, oriented, multilayer polyolefin film comprising a base layer which comprises polypropylene, and at least one non-heat-sealable outer layer. The films are distinguished by excellent antistatic properties in combination with excellent processing properties and a low coefficient of friction.
2. Description of Related Art
Transparent, oriented polypropylene films are employed, for example, in gloss lamination with paper or board. The films are generally non-heat-sealable, since the lamination is carried out by adhesive bonding of film and the paper or cardboard.
This application makes high demands on the film's optical properties and processing properties. The optical properties of the film are primarily described by the surface gloss and the haze. The processing properties of friction, antistatic characteristics, abrasion behavior, thickness profile, winding properties and flat lying of the film are of great importance.
Films having a low coefficient of friction are known. The demands on the processing properties of the films and their smooth passage through automatic machines have continually increased over the years. For this reason, ever-lower coefficients of friction are required. Today, the adjective "low" describes friction values which cover an order of magnitude of 0.3 to 0.1, whereas a few years ago a friction of 0.4 to 0.5 was regarded as extremely "low".
EP-A-0 124 310 describes films having low coefficients of friction which comprise a thick base layer and a thin outer layer containing finely divided inorganic particles. Inorganic particles of SiO.sub.2, aluminum silicates, sodium aluminum silicates and carbon black are disclosed. The particle size is in the range from 0.2 to 5.0 .mu.m. The particles have an advantageous effect on the coefficient of friction of the film.
EP-A-0 350 168 describes a multilayer film having differentiated surface-slip characteristics of the two surfaces. The outer layers are heat-sealable and contain SiO.sub.2 as antiblocking agent.
EP-A-0 234 758 describes a multilayer polyolefin film having a good absorption capacity for water-based coatings. The polypropylene outer layer contains an antiblocking agent and silicone oil. SiO.sub.2, silicates, chalk, clay and the like are disclosed as suitable antiblocking agents. However, there is no detailed information on the particle size of the various antiblocking agents.
DE-A-35 17 795 describes multilayer polypropylene films whose outer layer contains a combination of amine, polydialkylsiloxane and a platelet type of an inorganic pigment. The film is distinguished by good antiblocking and surface-slip characteristics. The platelet type of pigment has a leaf type of structure. Suitable pigments are silicates and carbonates.
By applying the known teaching, it has been found that conventional SiO.sub.2 in the production and further processing of the films results in deposits on the die lip and in considerable abrasion on the rolls over which the film passes during production and further processing. It has been found that this abrasion is caused by the SiO.sub.2 present in the outer layers. This abrasion results in a number of problems. The die lip and the rolls must be cleaned frequently, since the film otherwise runs poorly during production. The deposits on the die lip result in streaking on the film, which impairs the film's appearance. In addition, these impurities cause problems during corona treatment. The corona treatment breaks through in the areas of the roll where SiO.sub.2 abrasion has occurred and results in the undesired phenomenon known as the reverse-side effect. This reverse-side effect means that the reverse side of the film which has been corona or flame treated on one side also experiences treatment. These areas on the reverse side which have been unintentionally treated result in partial blocking and in adhesion of ink to the reverse side of the film in these areas during printing.
Furthermore, the films described in the prior art exhibit very poor roller make-up and do not lie flat. In addition, the antistatic properties are unsatisfactory.